The present invention relates to an apparatus for avoiding damaging bearing currents in an electrical machine, and to an electrical machine which is equipped with an apparatus such as this. The present invention also relates to a corresponding method for avoiding damaging bearing currents.
The aim is to avoid lubricating film discharges in bearings, as far as possible, such as those which can occur in steam or gas turbine rotors on bearings. Specifically, the friction of the fluid jet on the blades in these rotors results in electrostatic charging. Furthermore, the bearings of gearboxes, piston engines or other mechanical rotating machines are also intended to be protected. In addition, one aim is in principle to avoid damaging bearing currents in variable rotation speed machines.
Variable rotation speed motors are nowadays generally fed from voltage intermediate-circuit converters. The use of a voltage intermediate-circuit converter for feeding leads to bearing currents in the motor bearings. These bearing currents can lead to premature failure of the bearings, depending on the design of the motor. The failure is due to groove formation on the running surfaces of the bearing (vibration, noise) and to decomposition of the bearing grease.
In order to suppress bearing currents, so-called circulating currents, current-insulating bearings are frequently used, for example bearings with ceramic insulation of the outer ring or inner ring. Alternatively, hybrid bearings are used, with steel rings and ceramic roller bodies, in order to avoid circulating currents and EDM currents. However, these bearings are very expensive and are therefore avoided as much as possible. In addition, solutions for avoiding bearing currents are known in which the rotor is grounded by means of grounding brushes. However, the grounding brushes are subject to wear, and the contacts are not reliable, particularly in rough environmental conditions. In addition, in an “Industry White Paper” entitled “Inverter-Driven Induction Motors Shaft and Bearing Current Solutions”, the Rockwell Company have proposed that specific shielding be provided between the rotor and stator. In addition, converters with specific pulse patterns to reduce the bearing currents are also known for this purpose. All these solutions have the common feature that they are relatively expensive and complex.